1. Field of the Invention
The present invention relates to a pressure actuated switching device for closing or opening an electric circuit, and particularly to a safety mat for operating and shutting down machinery in response to personnel movement onto the mat.
2. Background of the Art
Pressure actuated electrical mat switches are known in the art. Typically, such mat switches are used as floor mats in the vicinity of machinery to open or close electrical circuits.
For example, a floor mat switch which opens an electrical circuit when stepped on may be used as a safety device to shut down machinery when a person walks into an unsafe area in the vicinity of the machinery. Conversely, the floor mat switch can be used to close a circuit and thereby keep machinery operating only when the person is standing in a safe area. Alternatively, the floor mat switch may be used to sound an alarm when stepped on, or to perform some like function.
U.S. Pat. No. 4,497,989 to Miller discloses an electric mat switch having a pair of outer wear layers, a pair of inner moisture barrier layers between the outer wear layers, and a separator layer between the moisture barrier layers.
U.S. Pat. No. 4,661,664 to Miller discloses a high sensitivity mat switch which includes outer sheets, an open work spacer sheet, conductive sheets interposed between the outer sheets on opposite sides of the spacer sheet for contacting on flexure through the spacer sheet, and a compressible deflection sheet interposed between one conductive sheet and the adjacent outer sheet, the deflection sheet being resiliently compressible for protrusion through the spacer sheet to contact the conductor sheets upon movement of the outer sheets toward each other.
U.S. Pat. No. 4,845,323 to Beggs discloses a flexible tactile switch for determining the presence or absence of weight, such as a person in a bed.
U.S. Pat. No. 5,019,950 to Johnson discloses a timed bedside night light combination that turns on a bedside lamp when a person steps on a mat adjacent to the bed and turns on a timer when the person steps off of the mat. The timer turns off the lamp after a predetermined period of time.
U.S. Pat. No. 5,264,824 to Hour discloses an audio emitting tread mat system.
While such mats have performed useful functions, there yet remains need of an improved safety mat which can respond not only to the presence of force, but also to the amount and direction of force applied thereto.
Also, mat switches currently being used often suffer from "dead zones". Dead zones are non-reactive areas in which an applied forced does not result in switching action. For example, the peripheral area around the edge of the conventionally used mats is usually a "dead zone". In the active area where switching does occur there is a danger of sparking when the two metallic conductor sheets touch. It would be advantageous to have a mat in which dead zones and sparking are reduced or eliminated.
Also known in the art are compressible piezoresistive materials which have electrical resistance which varies in accordance with the degree of compression of the material. Such piezoresistive materials are disclosed in U.S. Pat. Nos. 5,060,527, 4,951,985, and 4,172,216, for example.